Synchronous (full duplex) playback and recording means a situation when your ALSA card is opened for recording and playback at the same time. Examples of this could be, jackd running both input and output. An application which is used for recording and playing at the same time.

Known Bugs to be solved

Playback and recording (Asynchronous - full duplex)

An example of asynchronous (full duplex) playback and recording is when you use an application to play and then use a different application at a different time to record, without having the recording application synchronise with the playback applications. For example you may do the following : arecord | aplay. In this situation arecord starts before aplay this is asynchronous playback and recording. Most applications which play and record at the same time will implement synchronous full duplex playback and recording.